Lignin is a complex polymer made up of several aromatic rings linked together via C—C bond or C—O—C bonds (FIG. 1). Lignin is present in lignocellulosic materials with varying quantities (15-25%) depending upon the plant species, growth conditions etc. Upon depolymerisation, lignin can yield several aromatic monomers which can be used as fuel additives and platform chemicals. Generally, there are five categories of the chemical depolymerization of lignins reported in the prior art which includes (1) base-catalyzed, (2) acid-catalyzed, (3) metallic catalyzed, (4) ionic liquids-assisted, and (5) supercritical fluids-assisted lignin depolymerizations.
Article titled, “Depolymerization of oak wood lignin under mild conditions using the acidic ionic liquid 1-H-3-methylimidazolium chloride as both solvent and catalyst” by Blair J. Cox, John G. Ekerdt in Bioresource Technology 118 (2012) 584-588 reports the Oak wood lignin, which was separated from the wood using dissolution in the ionic liquid 1-methyl-3-ethylimidazoliumacetate and subsequent precipitation, was successfully depolymerized in the acidic ionic liquid 1-H-3-methylimidazolium chloride under mild conditions (110-150° C.). Based on gel permeation chromatography results, an increase in temperature from 110 to 150° C. increased the rate of reaction, but did not significantly change the final size of the lignin fragments.
Article titled, “Electro-catalytic oxidative cleavage of lignin in a protic ionic liquid” by Elena Reichert, Reiner Wintringer, Dietrich A. Volmerb and Rolf Hempelmann in Phys. Chem. Chem. Phys., 2012, 14, 5214-5221 reports a new approach of electro-oxidative cleavage of lignin, dissolved in a special protic ionic liquid, using an anode with particular electro-catalytic activity.
Article titled, “The effect of ionic liquid cation and anion combinations on the macromolecular structure of lignins” by Anthe George, Kim Tran, Trevor J. Morgan, Peter I. Benke, Cesar Berrueco, Esther Lorente, Ben C. Wu, Jay D. Keasling, Blake A. Simmons and Bradley M. Holmes in Green Chem., 2011, 13, 3375-3385 reports that sulfates>lactate>acetate>chlorides>phosphates in terms of the relative impact on reducing lignin molecular weight, with evidence of different anions causing cleavage of different linkages within the lignin. Of the ILs studied, sulfate based ionic liquids most comprehensively broke down the largest lignin molecules, resulting in fragments>1000-3000 u (by polysaccharide calibration). The lactate anion, while appearing less capable of breaking down the largest lignin molecules, causes the formation of significant quantities of the smallest sized fragments observed (2000-500 u).
Article titled, “Cleaving the (B—O-4) Bonds of Lignin Model Compounds in an Acidic Ionic Liquid, 1-H-3-Methylimidazolium Chloride: An Optional Strategy for the Degradation of Lignin” by Songyan Jia, Blair J. Cox, Xinwen Guo, Z. Conrad Zhang, and John G. Ekerdt in ChemSusChem 2010, 3, 1078-1084 reports The hydrolysis of B—O-4 bonds in two lignin model compounds was studied in an acidic ionic liquid, 1-H-3-methylimidazolium chloride. The B—O-4 bonds of both guaiacylglycerolb-guaiacyl ether and veratrylglycerol-b-guaiacyl ether underwent catalytic hydrolysis to produce guaiacol as the primary product with more than 70% yield at 150° C.
Article titled, “Hydrodeoxygenation of Lignin-Derived Phenols into Alkanes by Using Nanoparticle Catalysts Combined with Brønsted Acidic Ionic Liquids” by Ning Yan, Yuan Yuan, Ryan Dykeman, Yuan Kou, and Paul J. Dyson in Angew. Chem. Int. Ed. 2010, 49, 5549-5553 reports the transformation of lignin-derived phenolic compounds to alkanes has been achieved in ILs. The catalytic system is composed of metal NPs and a functionalized Brønsted acidic IL immobilized in a nonfunctionalized IL, allowing hydrogenation and dehydration reactions to occur in tandem.
Depolymerization of lignin yields products like gases (CO, H2, CH4, alkanes etc.) or substituted phenolic monomer compounds. Depolymerization of lignin using soluble base (NaOH, KOH, CsOH etc.) and using supercritical water is known. However, these methods are not environmentally benign and are energy consuming. Hence it is required to develop a process which will not use soluble base, will operate at milder reaction conditions, and will be environmentally benign. Despite the fact that some ionic liquids are appropriate solvents for lignin dissolution/depolymerization, however, the high cost of the ionic liquids limited their application on large quantity of lignin depolymerization reactions. Therefore, there is need in the art for recylization/recovery of ionic liquids. However, it is reported fact that it is difficult to separate the ionic liquid with lignin-derived molecules because of the π-π interaction between ionic liquid and aromatic moieties. Also, prior art failed to teach an effective method for recylization/recovery of ionic liquids.